Does a standard median coaddition of fits files in input_directory. Adapted from an example bash script found at http://montage.ipac.caltech.edu/docs/first_mosaic_tutorial.html :param input_directory: path to directory containing input images. :param output_directory: path to directory to write data products to; will be created if it doesn’t exist. :param output_file_name: Name of final coadded image file. :param ignore_differences: If False, checks if input images have compatible exposure times, filter, etc. and raises ValueError if not. :return: